Laser arrays show particular promise as illumination sources in imaging applications where brightness, high efficiency, and long component life are needed. As the cost of solid-state laser sources continues to drop and the available spectral range expands, solid-state laser arrays have been proposed as possible sources for digital projection and display apparatus, providing advantages over other types of light sources.
There are a number of applications for which a thin line of intense light is of particular interest. A number of spatial light modulators operate upon a single line of illumination and scan this line over a display surface for forming a two-dimensional image, for example. A general class of this type uses a linear array of micro-electromechanical modulators. Linear spatial light modulators of this type form images by a rapid, repeated sequence in which each single line of the image is separately formed and is directed to a screen or other display surface by reflection, or by other type of redirection, from a scanning element, such as a rotating mirror. Types of linear array light modulators that operate in this manner include devices such as grating light valve (GLV) designs, offered by Silicon Light Machines and described in U.S. Pat. No. 6,215,579 (Bloom et al.) and elsewhere. Display systems based on GLV devices are disclosed, for example, in U.S. Pat. No. 5,982,553 (Bloom et al.).
An improved type of linear array light modulator is the grating electromechanical system (GEMS) device, as disclosed in commonly-assigned U.S. Pat. No. 6,307,663 (Kowarz) and elsewhere. Display systems based on a linear array of conformal GEMS devices are described in commonly-assigned U.S. Pat. Nos. 6,411,425 and 6,476,848 (both by Kowarz et al.). Further detailed description of GEMS device architecture and operation is given in a number of commonly-assigned U.S. patents and published applications, including U.S. Pat. No. 6,663,788 (Kowarz et al.); and U.S. Pat. No. 6,802,613 (Agostinelli et al.). In GEMS devices, light is modulated by diffraction. On a GEMS chip, the linear array of conformal electromechanical ribbon elements, formed on a single substrate, provides one or more diffracted orders of light to form each line of pixels for line-scanned projection display.
GLV and GEMS color display system architectures generally employ three separate color paths, red, green, and blue (RGB), each color path provided with a linear array of electromechanical grating devices. Each linear array of electromechanical grating devices, when actuated, modulates its component red, green, or blue laser light to form a single line of the image at a time. The resulting modulated lines of light for each color are then combined onto the same output axis to provide a full-color image that is then scanned to the display screen.
In order to provide the best possible image quality when using linear spatial light modulators of this type, it is useful to provide a linear illumination that has these characteristics:                (i) Constrained spatial line width in the cross-array direction, with respect to a modulator for example. For GEMS devices, the linear illumination is a line of light (typically in the range of about 16 mm long) that is preferably no wider than about 100 μm. GLV devices require an even thinner spatial line width in the range of about 20 μm.        (ii) Uniformity. For intensity over the full length of the line of illumination that is provided, the line of illumination should be as uniform in intensity along the line as possible, without abrupt changes within that range.        (iii) Single-mode light in the width direction of the line of illumination. This characteristic relates to the requirements for diffractive order separation for GEMS light modulators. Single-mode operation provides modulated light having improved contrast.        (iv) Reduced speckle. Where possible, some amount of speckle reduction in the source illumination is advantageous. Arrays of uncorrelated emitters are advantaged for helping to reduce speckle effects when their light is combined.        (v) Telecentricity. Telecentric light is advantageous for electronic imaging applications in general. Where GEMS modulators are used, telecentricity is particularly advantaged, since it allows filtering of cross-order diffracted light in the array direction for improved contrast. Telecentricity of better than 4 mr (milliradians) in the array direction is needed.        (vi) Power. It is recognized that there are advantages to illumination solutions that can combine the light from two or more laser arrays to achieve higher power levels while maintaining good performance with respect to characteristics (i)-(v) noted above. A Gaussian distribution of intensity in the cross-array (width) direction is particularly desirable for GEMS modulation. This means preserving, as closely as possible, the original power distribution profile of the light that is emitted from the laser array itself. This is a difficult task, made more arduous as the light is processed by each successive optical component.        
Although conventional solutions have achieved some measure of acceptable performance for providing linear illumination, there is room for improvement with respect to each of characteristics (i) through (vi) just listed. Conventional solutions for providing linear illumination for GEMS devices have not taken advantage of laser arrays, but have employed single-mode, single-beam lasers for this purpose, with disappointing results. For example, maintaining the desired uniformity (ii) for a line of illumination of sufficient length proves to be a challenge when using single-beam lasers. Providing single-mode illumination (iii) is a challenge that is generally not recognized as a requirement with existing illumination systems. Speckle reduction (iv) is most advantageously addressed if speckle is corrected, at least somewhat, in the source illumination; it can be difficult to provide both single-mode illumination (iii) and reduced speckle (iv) at the same time. A disadvantage of single-beam lasers relates to power scaling; an incremental increase in output power can be disproportionately high in cost.
Laser arrays have been proposed for illumination with various types of two-dimensional spatial light modulators, such as micromirror-based devices, as described, for example, in U.S. Pat. No. 7,296,897 (Mooradian et al.). However, with this type of solution, the laser light from the array of sources is not provided as a thin line of light, but must be spread over a broad area, uniformly diffused over the full surface of the two-dimensional micromirror-based array.
The problem of providing a line of laser light that meets a portion of the basic requirements of at least characteristics (ii) and (v) given earlier was addressed for laser printhead applications using infrared light in commonly-assigned U.S. Pat. No. 6,137,631 (Moulin). In the Moulin '631 illumination system solution, a single laser array is used as the light source. An integrating element helps to uniformize light from multiple lasers in the array, which is then directed to a spatial light modulator, through a series of cylindrical lenses, and is focused by a printhead. Although this type of solution provides a line of light from a laser array, the characteristics of the light provided are not suitable for use with GEMS or GLV light modulators in display applications. For example, the Moulin '631 solution is unable to meet the constrained line width limits described earlier in (i) and, instead, provides as output spatial line widths that exceed those acceptable for GEMS and GLV devices. This solution is not suitable for highly coherent lasers. Further, no attempt is made to address speckle (iv). The Moulin '631 solution is constrained in terms of power output capability, limited to the output power that is available from a single laser array. Designed for printhead functions in the infrared spectrum, the Moulin solution would not be usable with GEMS- or GLV-based writers, display projectors, or other apparatus for example.
Earlier attempts to utilize laser arrays to provide some measure of uniformity fall short of achieving all of conditions (i)-(vi) noted above. As one example, U.S. Pat. No. 6,102,552 (Tullis) describes the use of an array of multiple lasers to direct uniform illumination toward a target, but does not form a line constrained in the cross-array direction as in (i) above and does not address single-mode illumination, speckle, telecentricity, or power distribution in a cross array direction as in (iii)-(vi) above. As another example, U.S. Pat. No. 5,517,359 (Gelbart) provides an optical arrangement using a lenslet array that superimposes light from each laser diode in an array in order to form a uniform line of illumination. However, the solution of the Gelbart '359 disclosure does not satisfy the line width requirements given in (i) above, nor address the need for single-mode light in the cross-array direction, reduced speckle, telecentricity of light, or the need for increased power using multiple arrays while preserving a suitable power distribution profile as in (iii)-(vi) above.
Thus, it is seen that there is a need for an apparatus that provides a uniform, thin line of light having single-mode characteristics in the cross-array direction and suitable power characteristics for display illumination and other applications.